docs: def cartesian(n, states):
      """ This function computes cartesians coordinates from the states returned by the simulate function.
  
      :param int n: number of particules suspended to the top one
      :param numpy.array states: list of the positions and speeds at a certain time.
      """
      length = 1. / n  # arm_length
      pos_x = hstack((states[0], zeros(n)))
      pos_y = zeros(n + 1)
      for j in arange(1, n + 1):
          pos_x[j] = pos_x[j - 1] + length * cos(states[j])
          pos_y[j] = pos_y[j - 1] + length * sin(states[j])
      pos = hstack((pos_x, pos_y))
      return pos